This essentially reverts a commit [1] that removed the adaptor for
Python unittests. The code has been slightly refactored to make it more
additive: all code is contained in LitTestCase.py.
Usage sites will require a small adaption:
```
[old]
import lit.discovery
...
test_suite = lit.discovery.load_test_suite(...)
[new]
import lit.LitTestCase
...
test_suite = lit.LitTestCase.load_test_suite(...)
```
This was put back on request by Daniel Dunbar, since I wrongly assumed
that the functionality is unused. At least llbuild still uses this [2].
[1] 70ca752ccf
[2] https://github.com/apple/swift-llbuild/blob/master/utils/Xcode/LitXCTestAdaptor/LitTests.py#L16
Reviewed By: ddunbar
Differential Revision: https://reviews.llvm.org/D69002
llvm-svn: 374947
74b285eee7